1. Field of the Invention
The invention relates to a control method for an industrial robot, in the following a robot, for the reduction of the influence of temperature changes on the positioning and orientation accuracy of the robot hand, in the following termed the tool center point (TCP) of the robot.
2. Description of the Prior Art
In the following, positioning accuracy is understood to mean the property of the robot to position the TCP at a pre-determined spatial coordinate x, y, z.
Orientation accuracy is understood to be the property to orient the TCP at a respective pre-determined angle ∠x, ∠y, ∠z at the spatial coordinate x, y z.
As accuracy requirements increase and stable and rugged process procedures are required, high demands come to be made with respect to the positioning and orientation accuracy. Particularly when a robot is in permanent operation, a drift in positioning and orientation accuracy can be determined whose cause is thermal sources which produce a deformation in the kinematic chain of the robot.
For technical reasons, it is not possible to completely prevent the temperature influence on the kinematic chain, since the drive assemblies of the kinematic chain generate heat under load, which results in changes in length and angle of the same in dependence on time. Nor can these effects be fully excluded by a thermo-symmetrical design.
To reduce positioning and orientation errors which arise due to the effect of temperature, the attempt is known from the prior art to measure the current temperatures at pre-determined points of components at selected points of the kinematic chain of a robot and to use these temperature measurement values for the correction of the position of the TCP by means of a computer.
For instance, a device is described in DE 33 02 063 for the compensation of influencing variables in machine tools or measuring machines and in robots. It is proposed to measure practically all the influencing variables which occur, such as temperatures and loads, and then to superimpose correction values on them which had been previously determined for all these influencing variables in order to thus achieve a compensation of the positioning error caused by the influencing variables.
The basic inventive idea of DE 33 02 063 accordingly comprises determining each influencing variable using a measuring device suitable for this purpose, i.e. the influencing variable xe2x80x9ctemperaturexe2x80x9d is determined by means of a temperature measuring device, the influencing variable xe2x80x9cdeformation by loadingxe2x80x9d is determined using a force measuring device, etc.
The inventors of the present invention have found, specifically for the correction of positioning and orientation errors arising due to temperature influences, that the method proposed in DE 33 02 063 is extremely complex and often cannot be carried out in practice. It has been found that the instruction given in DE 33 02 063 to arrange temperature sensors at selected points of a machine (in this case a robot) does not lead to the desired result since no sufficient correlation can be made between the temperature measured at the robot and the positional shift of the TCP which actually occurs with respect to the desired value.
For this reason, this compensation philosophy was not pursued further by the inventors of the present method.
Further devices for temperature compensation are described in the following documents.
U.S. Pat. No. 4,668,157 discloses a robot with a device for temperature compensation. The calibration cycle is triggered when the temperature at pre-determined points of the arm elements of the robot reach a pre-determined value. In this method, the current temperature at a pre-determined point at the robot arm is also put into correlation with the positional shift of the TCP.
U.S. Pat. No. 5,400,638 discloses a device for the calibration of a robot where thermally invariable reference bodies are used. The thermally induced changes in arm length are determined using the least squares method, with changes in angle being neglected.
It is the object of the invention to further improve the positioning and orientation accuracy of a robot in order to eliminate the above-mentioned problems.
The basic idea of the invention comprises an exact measurement of the tool center point (TCP) being carried out, preferably in the whole working area of the robot, in a zeroth method step. Temperature changes in the working area and in the robot itself are largely avoided here. In this measurement, the robot moves very slowly so that little heat is generated by the driving assemblies and the temperature gradients are as low as possible. The measurement can be made, for example, using a high-precision laser distance and angle measurement system. The measurement is carried out such that a measuring point is moved to working area points and then the deviations of the positions and/or orientations of the measuring point are determined using the laser distance and angle measurement system, i.e. a nominal/actual comparison is made. The TCP can preferably serve as the measuring point. However, a different point on the kinematic chain can also be selected. The point must always be selected so that the positioning and orientation deviations of the kinematic chain with respect to the PCT can be determined with sufficient accuracy.
The positioning and orientation errors determined in this measurement are termed stationary errors. A stationary error model, which forms a first basis for the correction of the temperature-induced deviations, is developed by known mathematical methods from the measurement of a great number of spatial points in the working area.
In the first, i.e. the next, method step, the same spatial points are again moved to as in the zeroth method step, with in particular the movement speed being substantially increased, whereby heat is generated in the drives of the kinematic chain and said heat results in the translatory and/or rotary deviations already described of the measuring point from the nominal value. These deviations Au are also stored, with a thermal error model, which describes the respective current thermal state of the kinematic chain, being prepared using mathematical methods known to one skilled in the art.
The zeroth and first method steps are usually only carried out once or at fairly large intervals, in particular after repairs.
In the second method step, a first subset U1 is determined from the set of spatial points measured. Only such spatial points are selected for this subset U1 which have such a temperature drift behavior which is representative for the temperature drift behavior of all spatial points; i.e. the temperature drift behavior of a point from the subset U1 is in each case in a predetermined proportion to the remaining spatial points. This predetermined proportion is determined in the next method step.
In the third method step, temperature-stable reference points RPU1 are arranged at the spatial coordinates of the subset U1 in the working area of the robot. A measuring device is fixedly arranged at the same position at the previously used measurement point and the positioning and orientation deviations of the measuring point are determinable with respect to the reference points RPU1 using said measuring device. The measuring device is moved to the reference points RPU1 between the working actions of the robot in accordance with a predetermined time sequence or predetermined criteria, with the current positional and orientation deviations being determined by means of a measurement of distance and angle from the respective reference point.
Temperature-stable bodies having measurement marks for the optical distance and/or angle measurement are used as reference points RPU1, for example. It is necessary to select those spatial points from the plurality of determined representative spatial points, which are suitable as reference points RPU1, at which the temperature-stable bodies do not impede the movement area of the robot and of the workpiece which is moved in and out. Systems which work in a non-contact method (such as optical methods) are suitable as measuring systems, as are tactile systems (such as inductive path measurement using a moving coil).
In the fourth method step, the current positioning and orientation errors between the measuring point and the reference points RPU1 are fed to the computer. The error model is adapted in the computer to the current thermal state of the kinematic chain of the robot on the basis of the measurement values determined and fed to the control electronics so that it is now again possible to move exactly to all points in the working area.
It is, however, clear that the error model must in particular supply an exact description of the kinematic chain of the robot for such regions in the working area in which the robot carries out the working actions.
This method has the following advantages over the methods known from the prior art:
it is usable in any type of robot;
it is simple;
for example, the extremely time-consuming search for correlations between the surface temperature of a component and the associated positioning and orientation error is dispensed with.
In a further development of the method, only the positioning errors are measured, i.e. the technical measurement effort is reduced; however, under certain circumstances also the accuracy of the error model.
In a further development of the method, only the orientation errors are measured, i.e. the technical measurement effort is reduced; however, also the accuracy of the error model.
In a further development of the method, the zeroth method step is not carried out and the measurement values to be determined are replaced by average values typical for the apparatus, whereby the working effort is reduced; however, under certain circumstances also the accuracy of the error correction.
In a further development of the method, the zeroth and first method steps are not carried out and the measurement values to be determined are replaced by average values typical for the apparatus, whereby the working effort is reduced; however, also the accuracy of the error correction.
In a further development of the method, a subset U2 is determined whose points only have a representative temperature drift for a selected region of the working area. This further development is expedient when the robot is always only in active movement in a partial region of the working area.
In a further development of the method, further subsets U3 to UN are determined whose points have a representative temperature drift for a selected respective region of the working area. This further development is expedient when the robot is in active movement in different, but predetermined, partial regions of the working area.
In a further development of the method, the measuring point is moved to the individual spatial points not only from one direction in the zeroth and first method steps, but from different directions. As each of the movement directions can result in a different deviation, more data is thus gained for the error model, whereby the error model becomes more accurate. The positioning and orientation accuracy of the robot can be further improved in this way.
In a further development of the method, such spatial points are selected from a subset which lie approximately on a straight line. It is possible with this measure to provide a very simple calibrating body, which lowers the device costs. In this case, a conventional precision rule is put into alignment with the straight line. Measurement marks are arranged on the precision rule and these are scanned by conventional measuring systems using a measuring head.
In a further development of the method, a sphere or a prismatic body is used as the reference body instead of the precision rule. Reference bodies of simple geometrical design can be produced with great precision at no great cost.
The further development of the method is to be preferred when the spatial points suitable as reference points do not lie on or in geometrically simple bodies. Accordingly, a temperature-stable wire is bent such that it extends through the spatial points suitable as reference points.
It is obvious to one skilled in the art that the reference bodies must always be arranged such that the work process is not impeded, i.e. that the reference bodies hinder neither the movement of the robot nor the movement of the workpieces or other devices.
The special advantage is to be emphasized that the method in accordance with the invention can be used irrespective of the constructive type of the robot. Furthermore, the invention can be used both with new robots and with those already in operation.